Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Common Stock and Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss, Net of Tax
Beginning balance (in shares) at Dec. 31, 2015		143,909,233
Beginning balance at Dec. 31, 2015	$ 3,122,950		$ 1,701,459	$ 1,872,594	$ (436,162)	$ (14,941)
Increase (Decrease) in Stockholders' Equity
Net income	320,943			320,943
Other comprehensive income	7,767					7,767
Stock compensation costs	13,973		13,973
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)		224,071
Net activity of common stock pursuant to various stock compensation plans and agreements	(163)		2,981
Net activity of common stock pursuant to various stock compensation plans and agreements					(3,144)
Cash dividends on common stock	(87,416)			(87,416)
Ending balance (in shares) at Sep. 30, 2016		144,133,304
Ending balance at Sep. 30, 2016	3,378,054		1,718,413	2,106,121	(439,306)	(7,174)
Beginning balance at Jun. 30, 2016						(1,712)
Increase (Decrease) in Stockholders' Equity
Net income	110,143
Other comprehensive income	(5,462)
Ending balance (in shares) at Sep. 30, 2016		144,133,304
Ending balance at Sep. 30, 2016	3,378,054		1,718,413	2,106,121	(439,306)	(7,174)
Beginning balance (in shares) at Dec. 31, 2016		144,167,451
Beginning balance at Dec. 31, 2016	3,427,741		1,727,598	2,187,676	(439,387)	(48,146)
Increase (Decrease) in Stockholders' Equity
Net income	420,726			420,726
Other comprehensive income	15,929					15,929
Stock compensation costs	15,780		15,780
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)		343,492
Net activity of common stock pursuant to various stock compensation plans and agreements	(10,695)		1,968
Net activity of common stock pursuant to various stock compensation plans and agreements					(12,663)
Cash dividends on common stock	(87,585)			(87,585)
Ending balance (in shares) at Sep. 30, 2017		144,510,943
Ending balance at Sep. 30, 2017	3,781,896		1,745,346	2,520,817	(452,050)	(32,217)
Beginning balance at Jun. 30, 2017						(34,181)
Increase (Decrease) in Stockholders' Equity
Net income	132,660
Other comprehensive income	1,964
Ending balance (in shares) at Sep. 30, 2017		144,510,943
Ending balance at Sep. 30, 2017	$ 3,781,896		$ 1,745,346	$ 2,520,817	$ (452,050)	$ (32,217)